Auxiliary power units (APUs) are commonly deployed on aircraft to provide an efficient source of electrical power, pressurized air, and/or hydraulic pressure. Among other components, an APU may include a gas turbine engine (GTE) having one or more fuel manifolds disposed within a combustion chamber. The fuel supply system includes, amongst other components, a fuel tank, a boost pump, a fuel supply pump, and a fuel metering valve. The manifolds include a series of nozzles (e.g., air blast nozzles and/or atomizer nozzles) that spray the burn fuel into the APU's combustion chamber. The resulting air-fuel mixture is then ignited to drive the rotation of one or more air turbines downstream of the combustion chamber. When including a GTE having multiple fuel manifolds (e.g., a primary manifold and a secondary manifold), the APU may be further equipped with a fuel divider system fluidly coupled between the fuel supply system and the GTE's manifolds. The fuel divider system apportions fuel between the primary and secondary manifold in accordance with a predetermined flow schedule (e.g., “quick fill logic”) to optimize GTE operation; e.g., to achieve optimal engine light-off conditions in a timely manner.
Due, at least in part, to recent regulations, it is now common for a GTE to be further equipped with an ecology valve (EV) fuel return system fluidly coupled to the GTE's fuel manifold or manifolds. The EV fuel return system is configured to remove a predetermined volume of burn fuel from the fuel manifolds upon cessation of GTE operation. Certain known piston/reservoir EV fuel return systems are further configured such that the withdrawn burn fuel is returned directly to the fuel manifolds for immediate combustion when GTE operation is again initiated (i.e., during engine start-up). Advantageously, by removing a predetermined volume of burn fuel from the fuel manifolds upon GTE shut-down, such piston/reservoir EV fuel return systems decrease the volume of fuel available for vaporization to the atmosphere and deter coking of the manifold nozzles. However, by returning this withdrawn burn fuel directly to the fuel manifold when GTE operation is again initiated, such piston/reservoir EV fuel return systems may disrupt the GTE's predetermined flow schedule and thereby comprise ideal engine light-off conditions.
Considering the above, it is desirable to provide an ecology valve fuel return system that avoids disruption of a predetermined flow schedule when returning fuel previously withdrawn from one or more fuel manifolds upon, or shortly after, GTE start-up. Ideally, such an EV fuel return system would be operable even when fluidly isolated from the fuel tank due to: (i) the closing of a shut-off valve between the EV fuel return system and the fuel tank, or (ii) high return pressures resulting from, for example, the inclusion of a relatively strong fuel boost pump within the fuel supply system. Other desirable features and characteristics of the present invention will become apparent from the subsequent Detailed Description and the appended claims, taken in conjunction with the accompanying drawings and this Background.